Four Major Characteristics of the Solar System

All planets and most satellites have nearly circular orbits in
nearly the same plane and revolve in the
same sense (counterclockwise when
looking down on the North Pole).

The Sun and most of the planets
rotate in this same sense as
well. (In other words, the rotation axis is roughly perpendicular to the
orbital plane. The deviation of the spin axis from the perpendicular is
called tilt or, more formally, obliquity.)

Definition of planet

On August 24, 2006 the International Astronomical Union (IAU) defined
the term "planet" for the first time. The definition was a controversial
one and has drawn both support and criticism from different astronomers.

The 2006 definition of "planet" by the IAU states that in the solar
system a planet is a celestial body that:

is in orbit around the Sun

has sufficient mass so that it achieves gravitational rounding (round
shape)

has "cleared the neighborhood" around its orbit.

A non-satellite body fulfilling only the first two of these criteria is
classified as a "dwarf planet", while a non-satellite body fulfilling only
the first criterion is termed a "small solar system body" (SSSB).

This definition excluded Pluto because its orbit is quite elliptical and
crosses that of Neptune. The IAU therefore reclassified Pluto as a member of the
new category of dwarf planets, along with Eris (previously known as 2003
UB313 or 'Xena') and Ceres (the largest asteroid).

Because of its orbital location, Pluto is now considered not only a dwarf planet but
also a Kuiper Belt object (though, for historical
reasons, will still occasionally be referred to as a planet). Click
here for a video
empathizing with Pluto's demotion. There is even
hate mail from
third graders.

According to the definition, there are currently eight planets and three
dwarf planets known in the solar system. The definition distinguishes
planets from smaller bodies, and is not used outside the solar system, where
smaller bodies cannot be found yet. Exoplanets are covered separately under
a complementary 2003 draft guideline for the definition of planets, which
distinguishes them from dwarf stars, which are larger.

Size comparison

The Sun has roughly 10 times the diameter of Jupiter

Jupiter has roughly 10 times the diameter of the Earth

The Earth has 5-6 times the diameter of Pluto.

Ceres, the largest asteroid, has a diameter roughly half that of Pluto.

Moons

A precise number of moons for the Jovian planets cannot be given, as
there is no objective boundary between the countless small anonymous objects
that form the ring system and the larger objects that have been named as
moons.

Jupiter's moon Ganymede is larger than Mercury and is the largest moon
in the solar system; Callisto is roughly the size of Mercury.

Saturn's moon Titan, the solar system's second largest moon (slightly smaller
than Ganymede and slightly larger than Mercury), is the only moon in the
solar system to possess a significant atmosphere, which is, in fact, denser
than ours.

Extra-solar systems

As of Oct 2014, at least 1763 extrasolar planets (exoplanets) have been
confirmed, with thousands more under investigation. For the latest on exoplanet research, visit
PlanetQuest.

None of the extra-solar systems discovered so far look much like our
solar system.

Most of the currently discovered exoplanets are massive giant planets
likely to resemble Jupiter.

In September of 2009, one exoplanet,
COROT-7b, was confirmed
to have Earthlike density.

Many of the extrasolar Jupiter-like giants orbit their parent star
within a fraction of 1 AU, hence the label "hot Jupiters".

The vast majority were detected through various indirect methods rather
than actual imaging.

Indirect observation typically involves the detection of a parent
star's wobble as the planet orbits it.

Wobble is most easily observed when the planet is large, so this
technique favors the discovery of large, massive planets.

Direct observation of exoplanets is challenging because of the
blinding glare of the parent star. This is analogous to the challenge of
seeing stars in our daytime sky.